Process for protection of chlorophyll during cooking of green vegetables



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United States Patent Ofiice Patented July 17, 1962 A non-exclusive, irrevocable, royalty freelicense in the invention herein described, for all governmental purposes, throughout the world, with the power to grant sublicenses for such purposes, is hereby granted to the Government of the United States ot-America.

This invention relates .to a process for protecting chlorophyll a from destruction during the-cooking of a green vegetable containing same thereby ito'rnaintain the :color of the vegetable using :the procedure in the article tentitled Stability of Ohlorophyllin VegetablesiasAlfected by pH by James P. Sweeney and Margaret E. Martin, appearing in Food Technology, vol. 15, No. 5, pp. 263-266, 1961.

.Approxirnately 75% of :the green coloring .matter .in plants, such as green vegetables, is in the form of the blue-greenchlorophyll a, the remainder being .chiefly the yellow-green chlorophyll b.

We have found that green vegetables, such as spinach, green peas, broccoli, Lima beans, green beans, and Brussels sprouts, differ greatly in their ability to retain chlorophyll a when cooked, and that the chlorophyll .a is :destroyed more rapidly than the chlorophyll b during the cooking process.

We have found further that an interrelationshipexists between the retention of the chlorophyll a and the :pH of the aqueous medium in which the green vegetable is cooked and that a pH ranging from 6.8 to 7.0 'is required [for maximum chlorophyll a retention in the resulting cooked vegetable.

According to the invention, we have discovered that such a pH range is produced and maintained in the vegetable and water, preferably in equal weights, when there is mixed therewith prior to cooking, citratephosphate buffer of-about pH 6.8. Such a butler, known as Mcllvaines bulfer, consists of 0.1 molar citric acid and 0.2 molar disodium phosphate, a bufier of pH 6.8 being prepared bymixing 4.55 ml. citric "acid with 15.45 ml. disodium phosphate.

As a result of such buffering of "the vegetableand water to a pH within the range 066.8% 70, maximum 2 Gardner automatic color difierence meter in which a Gardner mediumgreen color'standard,.L 5.0.0, 1;, 25.7. and 11. +7.3 were used. The chlorophyll .a retention values were determined bylthe-chromatographic.procedure described in the article entitled Determination of Chlorophyll and Pheophytin in Broccoli Heated by Various Procedures, by James P. Sweeney and Margaret Martin,

retention of'chlorophyll a is obtained thereby maintaining the color of the vegetable.

In carrying out the process of .the invention, .a green vegetable containing chlorophyll-aissoaked in -a solution of water and a suflicient quantity of 1a citrate-phosphate buffer, e.g., Mcllvaines' buffer, for about 10 minutes to produce and maintain a pH of 6.8 to 7.0 in the vegetable and then draining the resulting buffered solution "from "the vegetable. The vegetable .is subsequently-cookedin waterzcontainingabout 5 by volume, of the drained buifer solution, that is, about .5 ml. per 95 ml. of water, to obtain .a'cookedigreen yegetablein which a high percentage of the original chlorophyll .a has been retained.

We have found, also, '{that there-was a sharp decrease in panel flavor scores when such buffer of pH greater than pH 7 was employed. :On the other hand, such buifershavin'g :pHs inexcess of 7 caused relatively tlittle additional increase in green color, as indicated by 'a /b values. :In this ratio, which expresses .color, ithe a value is a measure of green reflected color and :bisa uneasure of yellow reflected color, readings being taken on a appearing in Food Research, vol. 23, No. 6, pp. 635 647, 1958.

It was found further that the buffer employed had no adverse effect on retention of ascorbic acid (vitamin C) in the vegetable.

The following examples are illustrative of the invention.

' EXAMPLE 1 In thisexample, chlorophyll a retention after cooking was determined on samples of each of 'frozenwegeltables, specifically, broccoli spears, Brussels-sprouts, cut :green beans, Fordhook .Lima beans, ;grean peas, and spinach.

In 'the'preparation :and cooking procedures, family-size packages of each brand .of the vegetableto'betested were removed from the freezer and opened. Those to be tested without cooking were allowed to thaw at room temperature "for about one :hour.

The samples to be cooked -,(:,tw.o' packages of each brand) were weighed. Apredetermined amountiof water (177 g. for spinach, :Lima beans, Brussels sprouts, :and broccoli; 188 g. *forp.eas;'-2-36.'g. for -green-.beans-) was brought to a e'boiling temperature, :as determined'yby thermocouple, in each of four 4-quart glass-covered saucepans. The unthawed vegetable was placed in the boiling water and timing was begun when -.the cooking water returned .to boiling. The cooking times selected were those-that had been shown by ,panel evaluation to result in vegetables of near optimum texture. Theresults areshown in the following Table 1. 7

- fable 1 EFFECT 'OFC'OOKIN'G TIME AND ,pH ON RETENTION Z'OF :OHLOiRORHYiLL a v 1 Cooking Ohlol'o hll Vegetable :time retailin (mim) as a 5 .(p'eroen'tl' 57850 .5. 76. .0 Spin h 10" "67.3 i 15 i6Z.=I1 .20, 156.6 0' 64.9 5 60.8 Green peas 10 60. 2 .15 56.6 .20 55.3 "0 06.9 -5 433:8 Brm'nnli 1O 51, 4 15 50.0 '20 3954 0 69.2 5- 434:3 Lima beans 10 54. 9 -15- 42.1 -20- aas 0- "76.5 -5- 157.1 Brusselsrsproutan, .10 5.0. 1 15 45.5 20 32.4 0 70.1 5 46.8 Greenbeansa", .10., 44.3 I 15" 41.1 1; I20? 27.?

The data of this table indicate 1am ichlorophyll a is destroyed when :the vegetable "is icooke'dyiand thatuthe (green beans and Brussels sprouts).

epaaaee 3 EXAMPLE 2 Experiments were also run to determine the elfect of pH on chlorophyll a retention. In this procedure, the vegetables were prepared by blending with an equal weight of distilled water. The pH readings were taken with a pH meter and the chlorophyll a retention values determined as for Example 1. The results obtained are summarized in the following Table 2.

Table 2 PERCENTAGE RETENTION OF CHLOROPHYLL IN FROZEN VEGETABLES AS AFFECTED BY pH l The cooking time in each case was that required for optimum texture.

2 Mean of 64 samples.

1 Mean of 4 samples for all vegetables except green beans; values for green beans, mean of samples.

As shown by Table 2, vegetables having the lowest pH lose chlorophyll a most rapidly when cooked. This is shown by the fact that cooked spinach and peas (pH 6.7) retained 77.2 and 67.6% of their chlorophyll a as compared to only 20.7 and 26.7% retention in Brussels sprouts and green beans at pH 6.2 and 6.0.

EXAMPLE 3 A study of the efiect of pH on color retention in frozen cut green beans was made. In this study, a modified cooking method was employed over that of Example 1. In this modified method, samples were allowed to thaw at room temperature and 70-gram portions were soaked for 10 minutes in the following solutions of chemical additives in amounts sufiicient to cover the vegetable: (1) distilled water; (2) 10% monosodium glutamate of pH 6.8; and (3) Mcllvaines citrate-phosphate buffer of pH 6.8 (described previously). The vegetables were drained, 5 ml. of the drained liquid added to 95 ml. boiling distilled water and the sample cooked in this liquid for 5 minutes. beans was evaluated by the Gardner automatic color difference meter (previously described), and pH measurements were taken on aqueous slurries.

The results are given in Table 3 hereafter:

Table 3 EFFECT OF CHELIICAL ADDITIVES OH COLOR RETENTION IN CUT GREEN BEANS WHEN COOKED An experiment, similar to that of Example 3, was carried out in which samples of green beans (French style) were soaked in water, bufiered with citrate-phosphate buffer at various pI-Is, namely, pH 6.2, 6.6, and 6.9, and then cooked as described in Example 3 in water to which 5 ml. of the buffered solution had been added.

The results are shown in Table 4 hereafter:

The a and b values of the cooked Table 4 EFFECT OF DH ADJUSTMENT ON RETENTION OF gggglgPHYLL a IN FROZEN GREEN BEANS WHEN pH of Chloro- Cooking Chemical added vegetable phyll rc (IL/[21, values time (min) slurry tained as a (Percent) None... 6. 1 57.0 0. 713 None... 6. 4 33. 9 0.57 Citrate-phos- 6. 2 3l. 9 0. 49

phate-bufl'cr, pH 6.8. -do 6. (i 42.0 0. 07 do 6. 9 50. 0 0. 71

The results shown in Table 4 indicate improved retention in color in cooked green beans with each increase in pH of the buffer. This is indicated by the higher retention of chlorophyll a (50% in green beans cooked in water buffered at pH 6.9 as compared to 31.9% buffered at 6.2) and by a corresponding increase in (I /b1, values to 0.71 from 0.49.

EXAMPLE 5 In further study on the effects of buffers ranging in pH from 6.4 to 8.90 on color and flavor of green beans, two l0-ounce packages of frozen green beans were allowed to thaw and were then immersed for 10 minutes in butters, drained, and cooked until done in 225 ml. boiling Water to which 25 ml. of the drained bulfer had been added. Buffers ranging from pH 6.40 to 8.90 were used. In addition, uncooked samples and samples cooked in water containing no added buffer were tested. Color difference meter and pH readings were taken as in Example 4. Flavor was evaluated by panel flavor scores, the flavor of the cooked vegetable being rated by a trained panel of five members, with a five point rating scale being used ranging from a score of 5 denoting natural full flavor to l denoting weak or lacking fiavor. The color was evaluated as previously described, with a Gardner automatic color difference meter.

The results are given in the following Table 5.

Table 5 EFFECT OF ph ADJUSTMENT ON COLOR AND FLAVOR OF FROZEN GREEN BEANS WHEN COOKED The results shown in this table indicate that there is a sharp decrease in panel flavor score when buffers of pH greater than 7 were used. On the other hand, buffers of pH more than 7 caused little additional increase in green color, as indicated by the a /b values.

EXAMPLE 6 In a further study on the effects of buffers ranging in pH from 6.40 to 8.40 on color and ascorbic acid retention of green beans, one pound of fresh green beans was cooked using the same procedure as that denoted in Example 5 above.

The results are given in Table 6 hereafter:

Table 6 EFFECT OF pH ADJUSTMENT ON COLOR AND ASCORBIC ACID IN FRESH GREEN BEANS WHEN COOKED pH of vege- Cooking table slurry an/br. value Asorblc time (min) pH of bufier acld (mg/100 g.)

No buffer Control (no bufier) 6. 40

The ascorbic acid reduction was determined by the procedure of Bessey and King, J. Biol. Chem., 103, 687 (1935).

As is apparent from the data of Table 6, buffer pHs ranging from 6.4 to 8.4 had no adverse efiect on ascorbic and retention.

We claim:

A process comprising soaking a green vegetable containing chlorophylla in a solution of water and a sufficient quantity of a citrate-phosphate b-ufier for about 10 minutes to produce and maintain a pH of 6.8 to 7.0 in the Vegetable, draining the bufiered solution from the vegetable, and subsequently cooking the vegetable in Water containing about 5%, by volume, .of the drained bufier solution, to obtain a cooked green vegetable in which a high percentage of the original chlorophyll a in the vegetable has been retained.

References Cited in the file of this patent Beeton: Book of Household Management, 1869, page 592.

De Gouy: The Gold Cook Book, 1947, page 697. 

